First reason to stick with the current method is consistency. If you change how you measure ice in and out then you can't compare dates anymore. Second is that this is simple, ice in is when the whole lake is covered, ice out is when all the ports are free. It's easy to check both. If you made ice-in when the ports are iced then you could get ice-in from a cold snap and then ice-out the next day when a warm wind turns up. The events that bracket the iced in period have to be well separated. There needs to be hysteresis.
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